Various types of downhole motors, including positive displacement motors and turbodrills may be suitable to drive a drill bit within a borehole, e.g. during drilling of the borehole. Steerable high speed motors, also known as turbodrills or turbines, are a commonly employed type of downhole motor and have become well known in the field of downhole drilling.
During the development of steerable high speed motors, it was recognised that at high speeds it was necessary that the motor and bit assembly be stabilised in order to reduce or eliminate wellbore tortuosity—commonly known as spiralling. This spiralling motion which can occur at high speeds can seriously reduce the drilling rate, as well as cause excessive wear of the various parts of the motor assembly. This spiralling effect can be particularly severe in the case of certain types of geological formations in which the bore is being formed.
In a typical drilling assembly the drill bit is connected to a motor shaft located inside a motor body.
The direction of formation of the wellbore may be controlled, e.g. by providing a bend, a deviating device, or an eccentric stabiliser, located at a suitable position of the assembly.
During normal drilling the motor body portion is rotated at a lower speed than the speed of the drill bit, thereby mitigating the effect of the deviating device. On the other hand, when directional or lateral drilling is required, the deviating device of the assembly is adjusted in a desired direction and held stationary, with the drill bit being rotated at high speed by the downhole motor.
In order to maximise the wellbore deviation, the so-called bit overhang (that is the distance from the lower end, e.g. lower bearing or lower stabiliser, on the motor body housing to the operating face of the drill bit) should be kept to a minimum.
Typically the majority of drill bits comprises a pin connection (Male) with an API thread to mate with a box connection (Female) API thread on the mating component, which may be a drill collar, sub or motor shaft. However, in turbine drilling it has become common practice for the thread connection to be reversed, the bit being provided with the box connection.
In downhole drilling, the terms “short gauge bits” or “long gauge bits” refer to the stabilising or guiding portion of the outer diameter that is used for the purposes of final trimming and guidance of the bit within the hole created by the bit. The gauge may include a sleeve to extend the guiding portion of the bit over a longer length. This sleeve can be made as an integral part of the bit structure. The extended sleeve portion typically has a diameter of +/− 1/32″ of the nominal bit diameter.
In the art, a short gauge bit is understood to mean a drill bit with an outer cylindrical portion the length of which measures approximately 1 inch to 1.0 times the nominal bit diameter. This contrasts with the so-called long gauge bits which may have cylindrical portions the lengths of which are in excess of 1 times the bit diameter. Furthermore, the so-called long gauge bits are often fabricated from separate pieces and have a short cylindrical portion, which forms part of the bit head and a second cylindrical portion formed from a separate sleeve and joined to the bit head. It is understood that the two cylindrical portions combine such that the cylindrical portion length is in excess of 1 times the nominal bit diameter. The two cylindrical portions are substantially of the same diameter but can be slightly different; approximately 1/32″ difference is possible due to normal manufacturing tolerance variations.
Short gauge bits have been used in drilling assemblies. However, known assemblies comprising a short gauge bit involved the use of a stabiliser between the gauge bit and the end of the motor body. While this type of arrangement is effective in stabilising the bit, the bit overhang is increased significantly thereby reducing the steerability of the motor assembly.
Current turbines tend to employ drilling bits having long total gauge lengths, typically from 1 times the nominal bit diameter to more than 2 times the nominal bit diameter. This has become necessary to ensure a smooth wellbore is produced. However, this introduces a risk that the drill bit may become stuck in the wellbore, and also increases the cost of the drill bit.
Recent developments in drill bits have led to motor assemblies which no longer require the presence of a bit box between the lower end of the motor shaft and the drill bit. Such an arrangement is described in U.S. Pat. No. 5,853,053 (GILCHRIST et al.). While the assembly disclosed therein provides a reduction in the bit overhang, some of the associated disadvantages may include premature wear of the stabiliser, and a relatively high risk that the long gauge drill bit may become stuck in the borehole.
It is an object of at least one embodiment of at least one aspect of the present invention to obviate and/or mitigate one or more disadvantages in the prior art.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a downhole drill motor stabiliser comprising reaming features or reaming means provided at or near at least a front portion of at least one blade of the stabiliser.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a drill bit comprising a gauge bit, e.g. a short gauge bit, and a connection means for connecting the drill bit to a drill motor assembly.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a downhole drilling assembly comprising an improved stabiliser, and optionally a drill bit such as a short gauge drill bit, and a motor assembly.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a locking means or lock and key mechanism for locking of a motor drive shaft through or together with a motor stabiliser, and beneficially allowing ease of handling, and attachment and/or removal of a drill bit to/from the motor drive shaft.
It is an object of at least one embodiment of at least one aspect of the present invention to provide a downhole drilling assembly comprising the locking means or lock and key mechanism.